A variety of high mobility/all-terrain vehicles have been made or proposed. These include a wide variety of tracked and wheeled vehicles.
A number of vehicles have been proposed for extraterrestrial use. Two which have been put into actual use are the four-wheeled lunar rover of the Apollo program and the six-wheeled robotic vehicle, Sojourner, of the Mars Pathfinder program. The suspensions of such vehicles are unidirectional; that is to say they function only when the vehicle is in a generally vertical orientation.
A variety of suspension systems may be used to maintain engagement of the vehicle's wheels with the ground surface. To ensure wheel engagement with particularly uneven surfaces, it is desirable that the suspension be mechanically actuated, either passively or actively, such as by hydraulics, pneumatics, motors or other actuators. In the case of a vehicle with more than three wheels, any three wheels will typically form a tripod upon which the vehicle may rest. To ensure engagement of the remaining wheels, it is necessary that the suspension have at least N-3 actuated degrees of freedom, where N is the number of wheels of the vehicle. If control over the ride height, and/or orientation are required then a system with N actuated degrees of freedom is necessary. The particular character of the actuated degrees of freedom will depend upon the chosen suspension design.
For example, the simplest of all designs would have one actuator vertically reciprocating each wheel. In the case of the Sojourner robot, each rear wheel is mounted on an arm which is pivotally actuated about a transverse axis of the robot. Each pair of front and middle wheels is carried by an arm extending between the axes of the wheels (a "rocker bogie"). The rocker bogie is pivotally moved on another arm which is pivotally actuated about the transverse axis of the robot. This provides the necessary three pivotal degrees of freedom on each side of the vehicle.
For maximizing traction, it is desirable that all wheels be driven. If individual motors are associated with each of the N wheels and individual actuators are associated with each of the N suspension degrees of freedom, then a total of 2N devices (actuators and motors) are required.